diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -493,7 +493,7 @@ def _xmerge(repo, mynode, orig, fcd, fco
         repo.ui.status(_('running merge tool %s for file %s\n') %
                        (tool, fcd.path()))
     repo.ui.debug('launching merge tool: %s\n' % cmd)
-    r = ui.system(cmd, cwd=repo.root, environ=env)
+    r = ui.system(cmd, cwd=repo.root, environ=env, blockedtag='mergetool')
     repo.ui.debug('merge tool returned: %s\n' % r)
     return True, r, False